The Conservation Law Foundation said it is planning to sue the Massachusetts Department of Transportation after officials illegally eliminated the Interstate 93 high-occupancy vehicle lane to allow all traffic to use the lane north of Boston. The HOV lane is a required component of CLF’s settlement with the state regarding mitigation from Big Dig emissions, and removing it violates the Clean Air Act and state law, the group said.

“With the daily traffic nightmare on our roads, opening this lane to all cars is short-sighted, slows commutes for bus riders, and violates the law,” CLF senior attorney Staci Rubin said in a statement. “Punishing people for carpooling and taking public transit is the exact opposite of good policy. For the sake of our environment and the health and well-being of residents north of Boston, the state must realize its mistake and undo this change immediately.”

The state opened the HOV lane to all traffic on May 14, arguing that construction and reduced lanes on the Tobin Bridge made the change necessary. But the carpoolers and bus riders who were previously given priority have experienced significantly longer commute times as a result, the foundation said.

CLF sued the state in the early 1990s to push for better transportation options as a result of the Big Dig. The settlement required a number of projects including the Green Line extension, the Red-Blue connector, and the creation and maintenance of high occupancy vehicle lanes.

CLF delivered the notice of intent to sue today; the state has 60 days to respond before the lawsuit is officially filed.

“MassDOT’s decision to eliminate priority for bus riders and carpoolers goes against its own stated goal of moving more people in fewer vehicles, and should be reversed immediately,” Chris Dempsey, director of Transportation for Massachusetts, said in a statement provided by CLF. “Decisions like this one have left Greater Boston with the worst traffic congestion in the country. We applaud the Conservation Law Foundation for taking legal action to protect commuters and the environment.”